The research program is directed toward a longitudinal appraisal of temperament in infant twins. Newborn twins offer a powerful resource for asessing the similarities and differences in temperament among genetically related pairs, and for examining the continuities in temperament over age. Twins also furnish a unique opportunity to observe infant-mother reciprocity in cases where the two infants may differ markedly in temperament, and thus evoke quite different reactions from the mother. The principal aims of the program are (a) to make a neonatal assessment of all twins before they leave THE HOSPITAL, FOR AN INITIAL MEASURE OF REACTIVITY/IRRITABILITY AND DEVELOPMENTAL STATUS; (b) to recruit a longitudinal sample of twins, and to perform a comprehensive appraisal of temperament at 3, 6, 9. 12, 18, 24 and 30 months, based on direct observations of the twins' behavior in a structured setting; (c) to supplement these observations with maternal interviews, and with temperament questionnaires (Carey et al.) completed for each twin by the parents; (d) to make home visits at 7 and 33 months, for an appraisal of the home environment and its contribution to the twins' development; and (e) to examine the role of temperament in shapping the social interaction of these twins, by measuring prosocial and agonistic behavior with twin pairs at 3 and 4 years of age. All sessions are videotaped, and rating scales have been developed that yield a composite temperament profile for each child. The results are utlimately addressed to questions of origins, patterning among variables, continuity/discontinuity over ages, and the joint collaboration of genetic and environmental factors as determinants of temperament. Temperament and personality measures are also obtained on the parents, to document parent-offspring relationships.